Conventionally, a water-absorbent resin has been widely used in hygienic materials such as disposable diaper and sanitary napkin, and industrial materials such as water blocking materials for cables. As the water-absorbent resin, there has been known, for example, hydrolysates of starch-acrylonitrile graftcopolymers, neutralized products of starch-acrylate graftpolymers, saponified products of vinyl acetate-acrylic ester copolymers, partially neutralized products of polyacrylic acid, and the like.
It has been desired that the water-absorbent resin used in hygienic materials or the like is excellent in properties such as water-retention capacity, water-absorption capacity under load, and gel strength. Also in the past, in order to improve the above-mentioned properties, many studies have been made on a method of increasing a crosslinking density on a surface layer of the water-absorbent resin (post-crosslinking method) and the like. For example, a method including the steps of mixing a carbonate compound such as ethylene carbonate and heat-treating the mixture (Patent Publication 1), a method including the steps of mixing a polyhydric alcohol compound and heat-treating the mixture (Patent Publication 2), and the like have been known.
On the other hand, since the water-absorbent resin is used in a hygienic material contacting to the human body (skin). Therefore, in recent years, it is desired that the water-absorbent resin gives consideration also to safety for the skin, and, as a method for improving the water-absorbent resin, a method which gives consideration to safety of the water-absorbent resin by using a crosslinking agent having high safety, reducing water-soluble substance, or the like tends to be studied.
In recent years, as a method for improving the above-mentioned properties while giving consideration to safety, for example, there has been suggested a method of increasing a crosslinking density on a surface layer of the water-absorbent resin, according to a method including the step of mixing with an oxetane compound and a water-soluble additive (see Patent Publication 3), a method including the steps of mixing with a ketal compound or an acetal compound and heat-treating the mixture (see Patent Publication 4), a method including the steps of mixing with a specified oxazoline compound and treating the mixture (see Patent Publication 5), or the like.    Patent Publication 1: German Patent No. 4020780    Patent Publication 2: Japanese Patent Laid-Open No. Sho 58-180233    Patent Publication 3: Japanese Patent Laid-Open No. 2003-313446    Patent Publication 4: Japanese Patent Laid-Open No. Hei 08-027278    Patent Publication 5: Japanese Patent Laid-Open No. 2000-197818